A spectrometer is an optical instrument that is used to measure properties of light, such as intensity, as a function of wavelength. Spectrometers are most often used in spectroscopic analysis to identify specific materials. By analyzing an object's light, physical properties of that object, such as temperature, mass, luminosity and composition, can be determined. Presently commercially available spectrometers typically combine a dispersive element, such as a diffraction grating, to separate a light beam into different component wavelengths, and one or more curved elements (reflective mirrors or lenses) that collimate and image the light from the dispersive element onto an elongated detector, such as a photosensor. The optical geometry of the system controls the size and shape of the light that is imaged onto the detector relative to its size and shape upon entry into the spectrometer. Light of different wavelengths is spread apart along the length of the detector.
One example of a currently available spectrometer is the Czerny-Turner spectrometer, which comprises a flat diffraction grating located between two spherically-curved concave mirrors. The first mirror collimates light from the entry spot onto the grating. The second mirror focuses light from the grating onto a detector, which in currently available spectrometers is typically a linear array of CCDs or other electronic light-sensors. The current spectrometers generally are either a “regular” Czerny-Turner spectrometer, in which the light is directed within the spectrometer housing along a generally M-shaped optical path, or a “crossed” Czerny-Turner spectrometer, in which the light is directed within the spectrometer such that it crosses over itself between the entry point and the detector..
In order to reflect the light beam within the housing, the curved mirrors are set off axis from the central ray of the light beam. This produces astigmatism in the reflected light beam. As a result, each color of light focuses to a line, rather than a spot. If the second mirror and the detector array are aligned for maximum resolution along the array, the line of light that results has a significant height perpendicular to the length of the array. If the height of the line of light is greater than the height of the sensors, the excess light is wasted and the sensitivity of the spectrometer is reduced.
To correct for such astigmatisms, one solution has been to modify a Czerny-Turner spectrometer to include a cylindrical lens extending along the detector array that will correct for the astigmatism and focus the line of light into a spot. A conventional cylindrical lens is designed to focus light much more strongly in one direction than in a perpendicular direction. Thus, it is used to focus the off-axis light onto the detector. It is even possible to use cylindrical lenses to simultaneously demagnify the light onto a detector array so as to focus the light on a narrower spot than the height of the detector that would otherwise result. However, it has been found that presently available cylindrical lenses focus the light perfectly at only one point along the array. That is, they are configured to optimize only one wavelength of light, typically at the center of the detector. Thus, conventional cylindrical lenses, while adjusting for some astigmatism, are not designed to correct astigmatism across substantially the entire spectrum and don't effectively demagnify across the full spectrum due to the astigmatism not being corrected fully as you move further away from the center.
The inventors also determined that no demagnification can be achieved without the presence of astigmatism. The only exception being a very limited demagnification can be achieved in a spectrometer as the result of using different arm lengths for the entrance and exit focal lengths. For example, a spectrometer with 100 mm input focal length and 75 mm output focal length demagnifies by ¾.
In the case of a Czerny-Turner spectrometer with corrected astigmatism (using an asphere/toroid focusing mirror, for example) or a concave aberration corrected grating spectrometer, the lack of astigmatism prevents the addition of a cylindrical lens for achieving demagnification. The inventors investigated various types of spectrometers that include astigmatism, and found that adding wedge cylindrical lenses simultaneously eliminates the astigmatism and induces demagnification across the full spectrum.
Current cylindrical lens corrections have been limited to use in Czerny-Turner type spectrometers since other types of spectrometers, such as concave grating spectrometers, correct the astigmatism with the grating and, therefore a cylindrical lens is not needed.